As young people spend more time on computers, smartphones and other devices, researchers are asking how all that screen time and multitasking affects children’s and teenagers’ ability to focus and learn — even drive cars.

A study from Stanford University, published Wednesday, wrestles with a new question: How is technology affecting their happiness and emotional development?

The answer, in the peer-reviewed study of the online habits of girls ages 8 to 12, is that those who say they spend considerable amounts of time using multimedia describe themselves in ways that suggest they are less happy and less socially comfortable than peers who say they spend less time on screens.